The present invention relates to a position actuator apparatus for environmental control devices such as valves, dampers and like control devices, and particularly such an actuator apparatus having a load responsive limit device for protecting the actuator and controlled device against abnormal force conditions which could cause damage, and more particularly to such an actuator apparatus having a force/torque limit device for signaling the presence of a load on the actuator and control device within predetermined limits regardless of dimensional variations within tolerances or dimensional changes of the parts over the life of the unit, such as rubber, seals, and with such predetermined limits provided in both directions, and further adapted to permit establishing of an open loop position control system.
The movement of mechanical devices such as valves, dampers and like flow control devices used in environmental air control system is provided through various drive units including pneumatic, hydraulic and electric driven actuators. Electric motor driven systems are well known for both large and relatively small load applications, such as positioning actuators in heating, ventilating and air conditioning systems, generally referred to as HVAC systems. HVAC systems use different cooling or heating mediums such as air and water with flow control valves, dampers and the like connected into the system for controlling and regulating the flow of the medium in response to various demand signals. Electric motor driven actuators are well known and widely used in HVAC systems The motors can be relatively small A.C. motors coupled to the valves or dampers through a mechanical force amplifying unit such as gears, lead screws or the like to match the drive force to the load.
The use of a small motor in combination with a load related gear or linkage provides a highly cost effective design for various sized loads. A single motor and an electronic control can also be readily adapted to various sized valve and damper specifications. A.C. synchronous motors and stepper motors provide a particularly satisfactory prime mover for HVAC systems because such motors can be used in either a closed loop or an open loop control system. In HVAC systems, the specifications as to positioning of the flow control device are relatively rigorous and demanding. The valve, damper and like devices as well as the motor will be damaged if over driven beyond the fully open and fully closed positions, or if some obstruction or mechanical failure prevents movement, with maximum motor driven force applied. The motor must provide a sufficient force or torque to overcome the load of the valve and/or damper, the interconnecting drive coupling mechanism and the load on the positioned device. Thus, the motor must also overcome any static forces created by the liquid or gas with the medium flowing through or applied to the positioned device. Generally, if maintained on the positioned device at its limit may significantly damage the motor, coupling or positioned device.
Generally, the prior art has provided various limit sensing devices for de-energizing of the motor with the positioned device at a limit position, or under an obstruction malfunction in a intermediate position.
The use of limit switches actuated at the opposite ends of the opening and closing of the positioned device requires careful and accurate positioning of the limit switches, with proper field calibration. An alternative control which tends to eliminate the disadvantage of position sensitive limit switches includes a force reaction device connected to the system wherein the load on the positioned device is monitored and in the event of abnormal force loading of the drive system, a switch is actuated and a signal is generated which is preferably connected to remove the power supply directly or indirectly from the motor. Such systems will respond to the loading of the drive mechanism in the intermediate position as well as the end positions and may be constructed with a fail-inposition system or with a return or reset to reference position system which can function as a fail safe control in selected applications. The drive mechanism can be constructed with a sufficient loading such as to hold the positioned device in its last position upon de-energization of the motor. Alternatively, the positioned device is readily provided with a reset spring unit which is loaded during movement from a reference position and returns the device to such reference on de-energization of the motor. The referencing position is usually either a fully closed or fully opened position, depending upon the particular required specifications. For example, in a winter heating system, a valve unit would generally be returned to a full haat position rather than a shut down position in order to prevent complete loss of heat during a cold condition.
The preventing of continued energization of the motor under limit load conditions is a highly significant requirement in HVAC systems and the like. Significant and expensive damage to costly components such as the motors, valves, dampers and the like will rapidly develop if the motor continues to operate at the limit positions and the like. Various force responsive systems have been suggested in the prior art. Typically, such systems are shown in U. S. Patents as follows:
______________________________________ U.S. Pat. No. Issue Date ______________________________________ 1,974,335 09-18-1934 2,407,537 09-10-1946 2,598,062 05-27-1952 2,763,797 09-18-1956 3,219,902 11-23-1965 3,430,916 03-04-1969 3,460,018 08-05-1959 4,114,078 09-12-1978 4,134,052 01-09-1979 4,265,270 05-05-1981 4,388,575 06-14-1983 4,595,081 06-17-1986 4,621,789 11-11-1986 ______________________________________
U.S. Pat. No. 2,763,797 which was issued on Sept. 18, 1965 discloses a system in which a preloaded spring unit is coupled in the drive train to actuate limit switches at the limit of the positioner An A.C. motor is coupled to drive a hollow shaft having a splined coupling to an output shaft. A worm on the output shaft is coupled to a worm gear for positioning of a valve, damper or the like. The output shaft is mounted for relative axial movement through the motor shaft, with a spring coupling permitting limited axial movement therebetween. The output shaft extends from the motor shaft, with an axially adjustable disk located between axially spaced limit switch units. The switch units are mounted in spaced relation to each other and to the opposite sides of the disk. Under normal operating conditions, the spring holds the output shaft in alignment within the hollow motor shaft to provide for simultaneous rotation of the worm and worm gear. If the worm gear is prevented from rotating, such as at the end of the position of a valve or damper, rotation of the worm creates a load back through the output shaft to the motor shaft and thus directly onto the motor. When the force is greater than the spring force, the worm rides on the fixed worm gear resulting in a movement of the output shaft against the force of the coupling spring. The result is movement of the output shaft and the switch disk secured thereto. Selected movement of the disk results in engagement with one of the switches which then deenergizes the motor. In such a system, the shaft and switch operating disk must move a predetermined amount directly related to the final drive coupling of the worm and worm gear in order to actuate the switches. Further, the switches as shown are separately and individually mounted much in the nature of limit switches. If a particular cutoff point is desired, field calibration may be required, whereas the device of the present invention has a cutoff point which eleminates field calibration. The elongated shaft connection of the worm and worm gear drive may introduce a certain degree of lost motion and varying tolerances, particularly with extended periods of use, may prevent repeatable and pinpoint response over the normal life of the system. Finally, the device is specifically constructed with the rotating worm and worm gear integrated telescoped shafts and is particularly applicable to spun-gear rotary devices and linear devices.
The A.C. synchronous motor and the stepper motor are desirable as being readily adapted to an open loop control system in contrast to a closed loop control system. An A.C. synchronous motor operates at a constant speed after initial start up. A.C. synchronous motors are also readily formed with bi-directional outputs by having appropriate forward and reverse run windings. Consequently, the position of the device can be monitored accurately by monitoring of the time and direction of A.C. motor operation. Further, A.C. synchronous motors are commercially mass produced and provide a cost effective source which has a long operating life when operated within load specifications. Relatively high speed, low torque A.C. motors can be used with appropriate load force amplifying gears, screws and the like mechanical connecting mechanisms. The direct position monitoring establishes a convenient open loop control.
Similarly, reversible stepping motors provide for precise movement of the load or positioned device for each step energization and step of the stepper motor. Again, by monitoring the number of steps and the direction of movement between the limit positions, a precise indication of the location of the positioned device is established. This again provides a cnnvenient open loop control system.
Although prior art actuators for HVAC systems include a mulitude of electric motor driven actuators as well as other forms of hydraulic and pneumatic actuators and many actuators are commercially available, the accuracy and reliability provided is often a compromise based on appropriate economic considerations of initial cost, maintenance cost and the like versus the essential or even necessary control requirements. The conflict and need of balancing cost requirements and operating specifications has established a continuing demand and need for a relatively low cost actuator of the force responsive type which meets the rigorous relationship between output displacement of the positioned device and the electrical input control. The demand is thus for a rapidly responding and accurate load responsive actuator which permits cost effective manufacture as by permitting reasonably wide manufacturing tolerances, produces a small, compact unit which can be coupled to the positioned valve, damper or the like in th minimum spaced requirements encountered in HVAC systems, is conveniently applied to different sized loads presented by the different positioned control devices in HVAC systems and can be used with both open and closed loop systems.
Environmental control systems include a sensor to produce a condition signal, such as temperature, to a controller which compares the condition signal with a set point signal and generates a control signal. The control valve or damper actuator is driven in accordance with a control signal. In a floating closed loop control, the control device is driven toward a fully open or fully closed position until the loop signals balance. In an open loop incremental system, the controller calculates the needed change in the control device position related to the difference between the sensor and setpoint signals and positions the control device accordingly. The open loop system is generally useful in applications in which a substantial plurality of different subsystems are controlled with a multiplexing of the subsystems. A third system provides a proportional control in which the position of the control device is continuously linearly positioned in accordance with the demand signal created by the difference in the sensor signal from the setpoint.